Substrate for a front-side-type image sensor and method for producing such a substrate

ABSTRACT

A substrate for a front-side type image sensor includes a supporting semiconductor substrate, an electrically insulating layer, and a silicon-germanium semiconductor layer, known as the active layer. The electrically insulating layer includes a stack of dielectric and metallic layers selected such that the reflectivity of the stack in a wavelength range of between 700 nm and 3 μm is greater than the reflectivity of a silicon oxide layer having a thickness equal to that of the stack. The substrate also comprises a silicon layer between the electrically insulating layer and the silicon-germanium active layer. The disclosure also relates to a method for the production of such a substrate.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No. 16/477,499, filed Jul. 11, 2019, now U.S. Pat. No. 11,282,889, issued on Mar. 22, 2022, which is a national phase entry under 35 U.S.C. § 371 of International Patent Application PCT/FR2018/050054, filed Jan. 10, 2018, designating the United States of America and published in French as International Patent Publication WO 2018/130781 A1 on Jul. 19, 2018, which claims the benefit under Article 8 of the Patent Cooperation Treaty to French Patent Application Ser. No. 1750235, filed Jan. 11, 2017.

TECHNICAL FIELD

The present disclosure relates to a substrate for a “front side” type imager, an imager incorporating such a substrate, and a method of manufacturing such a substrate.

BACKGROUND

The document US 2016/0118431 describes a “front side” type imager.

As illustrated in FIG. 1 , the imager comprises a SOI (Semiconductor-On-Insulator) type substrate comprising, from its rear side to its front side, a silicon support substrate 1′ having a certain doping, a silicon oxide layer 2′ designated BOX (buried oxide) layer, and a silicon layer 3′ designated active layer having a doping that can be different from that of the support substrate 1′, in which is defined a matrix array of photodiodes each defining a pixel.

However, such an imager has low sensitivity in the near infrared, that is to say for wavelengths comprised between 700 nm and 3 μm.

Indeed, the active silicon layer has a coefficient of absorption that decreases markedly with the wavelength of the radiation to which it is exposed, namely around 10⁶ cm⁻¹ for a wavelength of 300 nm to several 10³ cm⁻¹ from 700 nm.

BRIEF SUMMARY

An aim of the disclosure is to overcome the aforementioned problem and to propose a substrate for a “front side” type imager that makes it possible to increase the absorption of light in the near infrared.

To this end, the disclosure proposes a substrate for a front side type imager, successively comprising a semiconductor support substrate, an electrically insulating layer and a semiconductor layer designated active layer, made of silicon-germanium, characterized in that the electrically insulating layer is constituted of a stack of dielectric and metal layers chosen in such a way that the reflectivity of the stack in a range of wavelengths comprised between 700 nm and 3 μm is greater than the reflectivity of a silicon oxide layer having a thickness equal to that of the stack, the substrate further comprising a silicon layer between the electrically insulating layer and the active silicon-germanium layer.

“Front side” is taken to mean in the present text the side of the imager intended to be exposed to light radiation, which is located on the same side of the substrate as the associated electronic components.

Advantageously, the thickness of the electrically insulating layer is comprised between 10 and 500 nm.

According to one embodiment, the electrically insulating layer comprises at least one metal layer intercalated between a first dielectric layer situated at the interface with the support substrate and a second dielectric layer situated at the interface with the active layer.

The metal layer may be a titanium nitride layer, and the first and second dielectric layers may be silicon oxide layers.

According to one particular embodiment, the thickness of the first silicon oxide layer is comprised between 300 and 500 nm, the thickness of the titanium nitride layer is comprised between 10 and 100 nm, and the thickness of the second silicon oxide layer is comprised between 10 and 50 nm.

Preferably, the germanium content of the active layer is less than or equal to 10%.

Advantageously, the thickness of the active layer is less than a critical thickness of the silicon-germanium layer defined as being a thickness beyond which silicon-germanium relaxation takes place.

The disclosure also relates to a front side imager comprising such a substrate and a matrix array of photodiodes in the active layer of the substrate.

Another object of the disclosure relates to a method of manufacturing such a substrate.

The method comprises the following steps:

-   -   provision of a donor substrate comprising a semiconductor         material suitable to form the epitaxial growth of the active         layer, the semiconductor material being silicon;     -   provision of the support substrate; and     -   formation of the electrically insulating layer by deposition of         a stack of dielectric and metal layers on the support substrate,         the dielectric and metal layers being chosen in such a way that         the reflectivity of the stack in a range of wavelengths         comprised between 700 nm and 3 μm is greater than the         reflectivity of a silicon oxide layer having a thickness equal         to that of the stack;     -   bonding of the donor substrate on the support substrate, the         electrically insulating layer being at the bonding interface;     -   thinning of the donor substrate so as to transfer a silicon         layer onto the support substrate, and     -   epitaxial growth, on the transferred silicon layer, of a         monocrystalline silicon-germanium semiconductor layer to form         the active layer.

According to one embodiment, the stack of dielectric and metal layers deposited on the support substrate successively comprises a first dielectric layer, a metal layer and a second dielectric layer.

According to one embodiment, the metal layer is a titanium nitride layer, and the first and second dielectric layers are silicon oxide layers.

Advantageously, the thickness of the first silicon oxide layer is comprised between 300 and 500 nm, the thickness of the titanium nitride layer is comprised between 10 and 100 nm, and the thickness of the second silicon oxide layer is comprised between 10 and 50 nm.

According to a preferred embodiment, the method comprises a step of formation of an embrittlement zone in the donor substrate so as to delimit a layer of the semiconductor material to transfer onto the support substrate, and the thinning of the donor substrate comprises a detachment along the embrittlement zone.

Advantageously, the formation of the embrittlement zone comprises the following steps:

-   -   formation of a sacrificial oxide layer on the donor substrate,     -   implantation of atomic species in the donor substrate through         the sacrificial oxide layer, and     -   removal of the sacrificial oxide layer before the bonding of the         donor substrate on the support substrate.

The thickness of the silicon layer transferred onto the receiver substrate is typically less than or equal to 400 nm.

At the end of the epitaxial growth of silicon-germanium, the silicon layer may be conserved between the electrically insulating layer and the silicon-germanium layer.

Alternatively, the method may further comprise a step of condensation of the silicon-germanium of the active layer so as to convert the silicon layer, from which the epitaxial growth of silicon-germanium has been carried out, into a silicon-germanium layer.

According to another embodiment, the semiconductor material of the donor substrate suitable for the epitaxial growth of the active layer is silicon-germanium.

The semiconductor material is advantageously formed by epitaxy on a base substrate, the semiconductor material and the base substrate together forming the donor substrate.

BRIEF DESCRIPTION OF THE DRAWINGS

Other characteristics and advantages of the disclosure will become clear from reading the detailed description that follows, with reference to the appended drawings, in which:

FIG. 1 is a sectional view of a SOI substrate for a front side imager as described in the document US 2016/0118431;

FIG. 2 is a sectional view of a substrate for a front side imager wherein the electrically insulating layer is constituted of a stack of dielectric and metal layers;

FIG. 3 illustrates the reflectivity of a stack of dielectric layers as a function of wavelength (in nm);

FIGS. 4A-4E illustrate the main steps of a method of manufacturing the substrate for a front side imager according to one embodiment of the disclosure;

FIG. 5 shows the coefficient of absorption of silicon-germanium as a function of wavelength for different germanium contents;

FIG. 6 shows the critical thickness of a silicon-germanium layer as a function of the germanium content thereof;

FIGS. 7A-7F illustrate the main steps of a method of manufacturing a substrate for a front side imager according to one particular embodiment of the disclosure wherein the active layer is made of silicon-germanium;

FIGS. 8A-8C illustrate the steps of an alternative of the manufacturing method illustrated in FIGS. 7A to 7F;

FIG. 9 is a sectional view of a pixel of a “front side” type imager comprising a substrate according to one embodiment of the disclosure.

For reasons of legibility of the figures, the different layers are not necessarily represented to scale.

DETAILED DESCRIPTION

FIG. 2 is a sectional view of a substrate for a front side imager.

The substrate successively comprises, from its rear side to its front side, a semiconductor support substrate 1, an electrically insulating layer 2 and a monocrystalline semiconductor layer 3 designated active layer.

The support substrate 1 is generally obtained by cutting of a monocrystalline ingot. Advantageously, the support substrate 1 is made of silicon.

The active layer 3 is intended to receive a matrix array of photodiodes (not represented) enabling the capture of images. The thickness of the active layer 3 is typically greater than or equal to 1 μm. According to one embodiment, the active layer 3 is made of monocrystalline silicon. The active layer 3 may be lightly doped.

The electrically insulating layer 2 is not, as in conventional SOI substrates, a silicon oxide layer, but is constituted of a stack of dielectric and metal layers chosen so as to increase the reflectivity of the electrically insulating layer in the infrared.

More precisely, the dielectric and metal layers forming this stack are chosen in such a way that the reflectivity of the stack in a range of wavelengths comprised between 700 nm and 3 μm is greater than the reflectivity of a silicon oxide layer having a thickness equal to that of the stack.

Furthermore, a metal layer of the stack is advantageously separated from the support substrate and from the active layer by at least one dielectric layer. The dielectric layer ensures, in particular, a function of electrical insulation of the active layer vis-à-vis the support substrate.

For example, in the embodiment illustrated in FIG. 2 , the electrically insulating layer 2 successively comprises:

-   -   a first silicon oxide layer 21 at the interface with the support         substrate 1,     -   a titanium nitride layer 22, and     -   a second silicon oxide layer 23, at the interface with the         active layer 3.

Titanium nitride is a metal material widely used in microelectronics.

The dielectric silicon oxide layers 21 and 23 make it possible to encapsulate the titanium nitride layer and thereby avoid any metal contamination of the active layer. The generation of electrical defects at the interface between the active layer and the electrically insulating layer and re-combinations between the silicon of the active layer and the metal components of the imager, capable of doping the active layer, are thereby avoided.

Typically, the layer 21 has a thickness comprised between 300 and 500 nm, the layer 23 a thickness comprised between 10 and 50 nm, and the layer 22 a thickness comprised between 10 and 100 nm.

Such an electrically insulating layer has the advantage of reflecting more photons transmitted through the active layer 3 than a silicon oxide layer of identical thickness.

This is visible in FIG. 3 , which shows the reflectivity of the following stack (from the support substrate to the active layer) as a function of wavelength (in nm):

-   -   the first silicon oxide layer 21, of 400 nm thickness,     -   the titanium nitride layer 22, of 50 nm thickness, and     -   the second silicon oxide layer 23, of 20 nm thickness.

A reflectivity reaching 0.8 for a wavelength of 850 nm is thereby obtained. As a comparison, a 400 nm layer of silicon oxide has a reflectivity of 0.5 for a wavelength of 850 nm.

This increased reflectivity has the effect of increasing the length of the path of the photons within the active layer 3, and thereby of favoring their absorption by the layer.

Examples of methods of manufacturing the substrate illustrated in FIG. 2 will now be described.

Generally speaking, the method of manufacturing a substrate according to the disclosure comprises the following steps.

On the one hand, a donor substrate is provided comprising a semiconductor material suitable for the epitaxial growth of the material intended to form the active layer. Advantageously, the material is monocrystalline silicon. Alternatively, when the active layer has to be made of silicon-germanium (SiGe) as will be seen below, the material may be silicon-germanium (enabling homoepitaxy).

On the other hand, a receiver substrate is provided, and the donor substrate is bonded on the receiver substrate, an electrically insulating layer being at the bonding interface.

The donor substrate is next thinned so as to transfer a layer of the semiconductor material onto the receiver substrate.

This thinning may be carried out by polishing or etching of the semiconductor material so as to obtain the thickness and the surface state desired for the epitaxy of the material of the active layer.

However, advantageously, before the bonding step, an embrittlement zone is formed in the semiconductor material so as to delimit a superficial layer to transfer. Preferably, the formation of the embrittlement zone is achieved by implantation of atomic species, such as hydrogen and/or helium, in the donor substrate. After the bonding step, the thinning consists in detaching the donor substrate along the embrittlement zone, which leads to the transfer of the superficial layer onto the receiver substrate (method known as Smart Cut™). The thickness of the transferred layer is typically less than or equal to 400 nm. Potentially, a finishing treatment of the free surface of the transferred layer is carried out in order to favor the implementation of the epitaxy, the treatment being able to lead to thinning of the transferred layer.

In the case where such an implantation is implemented in the donor substrate, the stack of dielectric and metal layers is not formed on the donor substrate but on the support substrate. Indeed, such a stack would be too thick for the implantation. On the other hand, advantageously, before implantation, a sacrificial silicon oxide layer, through which the atomic species intended to form the embrittlement zone are implanted, is formed on the donor substrate. The sacrificial layer makes it possible to avoid any channeling effect during implantation. The sacrificial layer is next removed from the donor substrate before the bonding thereof on the support substrate.

Finally, on the transferred layer of semiconductor material, which serves as seed layer, the epitaxial growth of the material of the active layer is implemented until the desired thickness for the active layer is obtained.

FIGS. 4A to 4E illustrate one preferred embodiment of a substrate for a front side imager comprising such a stack of dielectric layers.

With reference to FIG. 4A, a donor silicon substrate 40 covered by a sacrificial silicon oxide layer 5 is provided, then an embrittlement zone 41 delimiting a silicon layer 42 to transfer is formed by implantation of atomic species through the sacrificial silicon oxide layer 5. After the implantation, the sacrificial silicon oxide layer 5 is removed, for example, by chemical etching.

With reference to FIG. 4B, a receiver substrate is moreover provided, which is typically the support substrate 1 of the final substrate, on which the stack of layers 21, 22, 23 intended to form the electrically insulating layer 2 is formed.

With reference to FIG. 4C, the donor silicon substrate 40 is bonded on the support substrate 1, the stack of dielectric and/or metal layers forming the electrically insulating layer 2 being at the bonding interface.

Next, the donor substrate is detached along the embrittlement zone 41. The detachment may be initiated by any technique known to those skilled in the art, such as a mechanical, chemical, and/or thermal stress.

The silicon layer 42 of the donor substrate is thereby transferred onto the support substrate 1 (cf. FIG. 4D).

If need be, a surface treatment of the transferred layer is carried out to remove defects linked to the implantation and to the detachment, and to make it sufficiently smooth for the epitaxy step that is going to follow.

With reference to FIG. 4E, epitaxy of the material of the active layer is then continued on the transferred silicon layer 42 which serves as seed layer, up to the desired thickness for the active layer 3. During epitaxy, it is possible to dope lightly the active layer 3, depending on the desired electrical properties.

The active layer may be made of monocrystalline silicon and the transferred layer serving as seed layer is then also made of monocrystalline silicon (homoepitaxy).

Particularly advantageously, the active layer is made of monocrystalline silicon-germanium, the transferred layer serving as seed layer being able to be made of monocrystalline silicon or monocrystalline SiGe.

A particular advantage of monocrystalline SiGe compared to monocrystalline silicon is its capacity to adsorb radiation in the near infrared.

As may be seen in FIG. 5 , which illustrates the coefficient of absorption (in cm⁻¹) of SiGe as a function of wavelength (in μm) for different compositions of the material, the coefficient of absorption, in particular, in the infrared, increases with the germanium content.

However, the design of the active layer 3 does not only concern the concentration of germanium but also the thickness of the layer. Indeed, since the SiGe layer is formed by epitaxy on a silicon substrate, the lattice parameter of which is different to that of silicon-germanium, relaxation of the SiGe layer takes place beyond a certain thickness, designated critical thickness. This relaxation results in the formation of dislocations within the SiGe layer.

Such dislocations would make the SiGe layer inappropriate for the function of the active layer 3 and must thus be avoided.

As shown in FIG. 6 , which illustrates the critical thickness (in Å) of a SiGe layer as a function of the germanium content (stoichiometric coefficient x corresponding to the composition Si_(1-x)Ge_(x)), the critical thickness is all the smaller the greater the germanium concentration.

The thickness of the active layer 3 and the germanium concentration of the layer thus result from a compromise between:

-   -   on the one hand, a sufficiently large thickness to capture a         maximum of photons in the wavelengths of the near infrared,     -   on the other hand, a sufficient concentration of germanium to         increase the capacity to absorb photons by the active layer, in         particular, in the near infrared, and     -   limited thickness (depending on the concentration) to avoid         silicon-germanium relaxation and the creation of crystalline         defects (dislocations) that result therefrom.

Typically, it is sought to maximize the thickness and the germanium concentration of the active layer 3 in order to have the best possible absorption in the infrared.

Preferably, the germanium content of the active layer is less than or equal to 10%. FIG. 6 shows in fact that the critical thickness of a Si_(0.9)Ge_(0.1) layer is of the order of a micrometer, which is suitable for the active layer of a “front side” type imager.

Thus, with the simultaneous use of the active silicon-germanium layer and the stack of dielectric and metal layers forming the electrically insulating layer, the favorable effects of these two layers on the absorption of infrared radiation are combined.

Compared to the manufacturing method that has just been described, the method of manufacturing a substrate in which the active layer is made of SiGe necessitates the provision of a donor substrate comprising a semiconductor material suitable for the epitaxial growth of silicon-germanium. The material may, in particular, be SiGe (enabling homoepitaxy) or a material different to SiGe but having a lattice parameter sufficiently close to that of SiGe to enable the epitaxial growth thereof (heteroepitaxy). In this latter case, the semiconductor material is advantageously silicon.

It will be noted that, when the seed layer is not made of SiGe, for example, when it is made of silicon, at the end of the epitaxy of SiGe, the silicon layer 42 under the active layer 3 remains.

However, the seed layer is sufficiently thin (of a thickness less than or equal to 300 nm) compared to the thickness of the active layer so as not to affect notably the properties of the active SiGe layer in terms of absorption in the infrared.

However, it is possible to remove the seed layer, for example, by means of a condensation method. In a manner known per se, the method comprises an oxidation of the SiGe layer, the oxidation having the effect of consuming only the silicon (to form silicon oxide) and to make the germanium migrate to the face opposite to the free surface of the SiGe layer. A SiO₂ layer, which can be removed by etching, is then obtained on the surface.

According to one embodiment, illustrated in FIGS. 7A to 7F, the starting point is a donor substrate 30 comprising a superficial SiGe layer 31.

The SiGe layer is typically formed by epitaxy on a base substrate 32, generally made of silicon. The SiGe layer is sufficiently thin to be stressed.

In a first version of this embodiment, an embrittlement zone is formed in the SiGe layer.

In a particularly advantageous manner, as illustrated in FIG. 7B, the embrittlement zone 33 is formed by implantation of atomic species (typically, hydrogen and/or helium) through the free surface of the SiGe layer 31. The embrittlement zone 33 thereby delimits a SiGe layer 34 at the surface of the donor substrate.

With reference to FIG. 7C, a receiver substrate is moreover provided comprising a support substrate 1 and the electrically insulating layer 2 constituted of the stack of dielectric and/or metal layers (to simplify the drawings, the layer 2 is represented as a single layer, without distinguishing the different constituent layers thereof).

With reference to FIG. 7D, the donor substrate is bonded on the receiver substrate, the SiGe layer 31 and the electrically insulating layer 2 being at the bonding interface.

Next, as illustrated in FIG. 7E, the donor substrate is detached along the embrittlement zone. The detachment may be initiated by any technique known to those skilled in the art, such as a mechanical, chemical, and/or thermal stress.

The SiGe layer 34 is thereby transferred onto the support substrate.

If need be, a surface treatment of the SiGe layer is carried out to remove defects linked to the implantation and to the detachment, and to make it sufficiently smooth for the epitaxy step that is going to follow.

In a second version of this embodiment, an embrittlement zone 33 is formed in the donor substrate 30 situated under the SiGe layer 31 (cf. FIG. 8A).

In a particularly advantageous manner, the embrittlement zone 33 is formed by implantation of atomic species (typically, hydrogen and/or helium) through the free surface of the SiGe layer 31. The embrittlement zone 33 thereby delimits a SiGe layer and a portion 38 of the base substrate 32 on the surface of the donor substrate.

With reference to FIG. 7C, a receiver substrate comprising a support substrate 1 and an electrically insulating layer 2 is moreover provided.

With reference to FIG. 8B, the donor substrate is bonded on the receiver substrate, the SiGe layer 31 and the electrically insulating layer 2 being at the bonding interface.

Next, the donor substrate is detached along the embrittlement zone 33. The detachment may be initiated by any technique known to those skilled in the art, such as a mechanical, chemical, and/or thermal stress.

The SiGe layer 31 and the portion 38 of the base substrate are thereby transferred onto the support substrate (cf. FIG. 8C).

A treatment of the surface created is then carried out to remove the portion 38 of the superficial donor substrate until a surface of SiGe is revealed, thereby removing defects linked to the implantation and to the detachment, and making it sufficiently smooth for the epitaxy step that is going to follow.

As in FIG. 7E, a portion 38 of the SiGe layer 31 on the support substrate 1 is thereby obtained.

As illustrated in FIG. 7F (step common to the two versions of the embodiment), epitaxy is then resumed in order to make a SiGe layer 35 grow on the transferred SiGe layer 34 which fulfils the role of seed layer, up to the desired thickness for the active layer 3, which is formed of both of the two SiGe layers 34 and 35 together. During epitaxy, it is possible to dope lightly the SiGe layer 35, depending on the desired electrical properties. The doping of the SiGe layer 35 is not necessarily identical to that of the seed layer 34.

FIG. 9 illustrates a part of a front side type imager comprising a substrate according to one embodiment of the disclosure. Only a part of the imager corresponding to a pixel is represented in this FIG., the pixel being electrically insulated from the other pixels formed in the active layer 3 by insulation trenches 7.

A doped region 36 of type different to that of the active layer 3 is formed under the surface of the front side of the active layer 3. This region 36 forms with the active layer 3 a photodiode. A region 37 formed between the region 36 and the front side of the active layer 3 advantageously has a doping level greater than that of the region 36 in order to passivate the interface. A passivation layer 6 is formed on the active layer 3 and may encapsulate elements making it possible to control the pixel electrically.

Potentially, other layers, such as filters, may be formed on the passivation layer 6, but they are not represented in FIG. 9 .

The structure of the imager as such and its method of manufacture are known to those skilled in the art and will thus not be described in detail herein. 

What is claimed is:
 1. A substrate for a front side imager, comprising: a semiconductor support substrate; an electrically insulating layer comprising at least one metal layer, a thickness of the at least one metal layer is between 10 and 100 nm; a semiconductor active layer comprising silicon-germanium; and a silicon layer between the electrically insulating layer and the semiconductor active layer.
 2. The substrate of claim 1, wherein the electrically insulating layer further comprises a stack of dielectric layers and the at least one metal layer, a reflectivity of the stack in a range of wavelengths between 700 nm and 3 μm being greater than a reflectivity of a silicon oxide layer having a thickness equal to that of the stack.
 3. The substrate of claim 1, wherein the electrically insulating layer further comprises a first dielectric layer and a second dielectric layer.
 4. The substrate of claim 3, wherein a thickness of the first dielectric layer is between 300 and 500 nm.
 5. The substrate of claim 3, wherein a thickness of the second dielectric layer is between 10 and 50 nm.
 6. The substrate of claim 3, wherein the first dielectric layer comprises a first silicon oxide layer, and the second dielectric layer comprises a second silicon oxide layer.
 7. The substrate of claim 1, wherein the electrically insulating layer comprises the at least one metal layer, a first dielectric layer, and a second dielectric layer, the at least one metal layer is intercalated between the first dielectric layer and the second dielectric layer, the first dielectric layer is situated at an interface with the support substrate, and the second dielectric layer is situated at an interface with the silicon layer.
 8. The substrate of claim 1, wherein the germanium content of the semiconductor active layer is less than or equal to 10%.
 9. The substrate of claim 1, wherein the germanium content of the semiconductor active layer is less than or equal to 8%.
 10. The substrate of claim 1, further comprising a bonding interface, the electrically insulating layer being present at the bonding interface.
 11. The substrate of claim 1, wherein a thickness of the silicon-germanium layer is greater than 1 μm.
 12. A substrate for a front side type imager, comprising: a semiconductor support substrate; an electrically insulating layer comprising a stack of dielectric and metal layers, a reflectivity of the stack in a range of wavelengths between 700 nm and 3 μm being greater than a reflectivity of a silicon oxide layer having a thickness equal to that of the stack; a bonding interface, the electrically insulating layer being present at the bonding interface; a semiconductor active layer comprising silicon-germanium; and a silicon layer between the electrically insulating layer and the semiconductor active layer.
 13. The substrate of claim 12, wherein the electrically insulating layer comprises at least one metal layer, a first dielectric layer, and a second dielectric layer, the at least one metal layer intercalated between the first dielectric layer and the second dielectric layer, the first dielectric layer is situated at an interface with the support substrate, and the second dielectric layer is situated at an interface with the silicon layer.
 14. The substrate of claim 12, wherein a thickness of the electrically insulating layer is between 10 nm and 500 nm.
 15. The substrate of claim 12, wherein a thickness of the electrically insulating layer is between 10 and 200 nm.
 16. The substrate of claim 12, wherein the metal layer comprises a titanium nitride layer.
 17. The substrate of claim 12, wherein the reflectivity of the stack for a wavelength of 850 nm is 0.8 or more.
 18. The substrate of claim 12, wherein a thickness of the semiconductor active layer is less than a critical thickness of the silicon-germanium layer defined as being a thickness beyond which silicon-germanium relaxation takes place.
 19. The substrate of claim 18, wherein the thickness of the semiconductor active layer is greater than 1 μm.
 20. A front-side imager, comprising: a substrate, successively comprising: a semiconductor support substrate; an electrically insulating layer comprising at least one metal layer, a thickness of the at least one metal layer is between 10 and 100 nm; a semiconductor active layer comprising silicon-germanium; a silicon layer between the electrically insulating layer and the semiconductor active layer; and a matrix array of photodiodes in the active layer of the substrate. 